It is quite easy, pretty similar to making jelly anyway. You need to heat up the juice, add the gelatine crystals (the sheets are a bit trickier to use I find), let them disolve and then chill until set. Some fruit if added stop it setting as well, the more acidic ones like pineapple for example.
The best results I've had have been with quite thick mango juice.
I've also made it with ribeena before but I guess that's full of crap!